Aluminum oxide is an electrical insulator and is transparent to visible light. It is a strong, hard material and resists attack by most chemicals. Aluminum oxide layers form good barriers against diffusion of many materials, such as sodium. Aluminum oxide may be formed by reactions of a wide variety of aluminum-containing compounds in processes called chemical vapor deposition (CVD), in which an aluminum-containing vapor reacts on a hot surface to deposit aluminum oxide.
CVD of aluminum oxide has been demonstrated from trialkylaluminum precursors, which have the general formula AlR.sub.3, in which R stands for an organic radical, such as methyl, ethyl, isopropyl, etc. Trimethylaluminum and triethylaluminum are precursors in this class. If vapors of trimethylaluminum are mixed with low concentrations of oxygen near a heated surface, a layer of aluminum oxide is deposited. This process is described by K. M. Gustin and R. G. Gordon in the Journal of Electronic Materials, Volume 17, pages 509-517 in 1988. Aluminum alkyls have some disadvantages in a CVD process. Aluminum alkyls ignite spontaneously in air, so that they are a serious fire hazard. Larger amounts of oxygen in the CVD chamber cause powdered aluminum oxide to form, instead of films. Thus the CVD processes for forming aluminum oxide films from aluminum alkyls can be disrupted by air leaks into the CVD chamber.
Aluminum 2,4-pentanedionate (also known as aluminum acetylacetonate) is another precursor used for CVD of aluminum oxide. It has the advantage over the aluminum alkyls of not being pyrophoric, and in fact it is completely stable in the presence of air and water at ambient temperatures. Aluminum 2,4-pentanedionate may be vaporized from its melt in a bubbler at temperatures above its melting point of 189.degree. C., the vapors mixed with dry oxygen gas, and passed over a heated substrate, in order to deposit films of aluminum oxide. This process is described by R. G. Gordon in U.S. Pat. No. 4,308,316 (1981). A disadvantage of using aluminum 2,4-pentanedionate is that it decomposes when the material is heated for several hours in a bubbler, so that successive deposition runs do not have the same vapor concentration or deposition rate.
Aluminum isopropoxide has also been used to deposit aluminum oxide films. See, for example, J. A. Aboaf in the Journal of the Electrochemical Society, volume 114, pages 948-952 (1967). This material exists in a number of isomeric forms, ranging from dimers to trimers to polymers of various lengths. The rates of interconversion between isomeric forms are slow, often taking days. The vapor pressures of these isomers vary widely. Thus it is very difficult to regulate or predict the vapor pressure of any particular sample of aluminum isopropoxide, and the deposition rate of aluminum oxide is not reproducible.
Aluminum 2-ethylhexanoate has been proposed as another source for CVD of aluminum oxide, by T. Maruyama and T. Nakai in Applied Physics Letters, volume 58, pages 2079-2080 (1991). This solid source material has a very low vapor pressure, which limits the deposition rate to low values.
These aluminum oxide precursors are all solids at room temperature, except for the aluminum alkyls, which are normally liquids. Solid materials are less convenient to handle and move into bubblers, than are liquid precursors. Another advantage of liquid precursors is that they may be vaporized by directly injecting a controlled flow of the liquid into a preheated carrier gas. This direct liquid injection method for vaporization is becoming increasingly widely used in CVD processes, because it allows very reproducible control of the vapor concentration, and minimizes or eliminates premature decomposition of the precursor materials. Thus another disadvantage of the solids, aluminum 2,4-pentanedionate, aluminum isopropoxide and aluminum 2-ethylhexanoate, is that they cannot be vaporized by the direct liquid injection method.
Dialkylaluminum alkoxides have been shown to dope crystalline III-V semiconductors such as GaAs with small amounts of oxygen (parts per million), by M. S. Goorsky, T. F. Kuech, F. Cardone, P. M. Mooney, G. J. Scilla and R. M. Potemski, Appi. Phys. Lett., vol. 58, pages 1979-1981 (1991); T. F. Kuech and M. A. Tischler, U.S. Pat. No. 5,098,857 (1992); J. W. Huang and T. F. Kuech, Appl. Phys. Lett., vol. 65, pages 604-606 (1994); J. W. Huang, D. F. Gaines, T. F. Kuech, R. M. Potemski and F. Cardone, J. Electronic Mater., vol. 23, p. 659 (1994); and J. W. Huang, T. F. Kuech and T. J. Anderson, Appl. Phys. Lett., vol. 67, pages 1116-1118 (1995). These references do not teach or suggest that alkylaluminum alkoxides could be used to deposit pure aluminum oxide films. In fact, the Appl. Phys. Lett., vol. 65 (1994) publication states that diethylaluminum ethoxide has a suitable vapor pressure for use as a dopant, which would lead a one to expect that it has a vapor pressure too low to act as a CVD source for a film of pure aluminum oxide. None of these references suggest that any alkylaluminum alkoxide could be homogeneously mixed with oxygen in a gas mixture for depositing aluminum oxide. In fact, the Appl. Phys. Lett., vol. 58, publication states that O.sub.2 would not be incorporated readily in films grown under their conditions.